1. Field of the Invention
This invention relates to an optical pickup device for writing and reading information signal into/from a magnetic optical recording medium such as an optical disk and magnetic optical disk, and a disk player apparatus for recording and reproducing information signal in the optical disk or magnetic optical disk including the aforementioned optical pickup device.
2. Description of the Related Art
Conventionally, various types of optical recording mediums such as an optical disk (bit recording disk, phase change type disk, rewrite type disk, etc.) and magnetic optical disk have been proposed. Such an optical recording medium includes a transparent substrate and a signal recording layer formed thereon. In the optical disk and magnetic optical disk, its substrate is formed in the form of a circular disk. In the optical disk or magnetic optical disk, information signal is recorded in recording tracks formed in substantially coaxial spiral shape, in the signal recording layer.
An optical pickup device for writing and reading information signal into/from a magnetic optical disk 101 which is an optical recording medium, as shown in FIG. 1, has been proposed. This optical pickup device includes a semiconductor laser 201 as a light source. Beam emitted from the semiconductor laser 201 is converged and irradiated to a signal recording surface of the magnetic optical disk 101 by means of an objective lens 205, that is, to the surface of a signal recording layer 102. Beam emitted from the semiconductor laser 201, passing through a grating (diffraction grating) 202, a beam splitter 203 and a collimator lens 204, is introduced to an objective lens 205. The grating 202 enables detection of tracking error signal which will be described later.
In this optical pickup device, by detecting beam irradiated to the signal recording surface and then reflected thereby by means of an optical detector (P.D.: photo diode) 209, information signal recorded in the signal recording layer 102 of the magnetic optical disk 101 is read out and further error signals, that is, focus error signal and tracking error signal are detected in order to maintain convergence of beam upon the signal recording surface.
Reflected beam, passing through the objective lens 205 and the collimator lens 204, returns to the beam splitter 203. This reflected beam is reflected by the beam splitter 203 and, passing through a Wollaston prism 207 and a multi-lens 208, impinges upon an optical detector 209. The Wollaston prism 207 divides beam corresponding to polarization component of the impinging beam. The multi-lens 208 has a cylindrical surface as an incident surface, and a concave surface as an emission surface. This lens causes astigmatism for detecting the focus error signal, in the impinging beam, and retracts a converging point for the incident beam.
The focus error signal is a signal for indicating a distance between a convergent point of beam and the signal recording surface, in the direction of optical axis of the objective lens 205. In the optical pickup device, as indicated by the arrow F in FIG. 1, the objective lens 205 is moved in the direction of the optical axis thereof until the focus error signal becomes 0, that is, focus servo action is conducted.
The tracking error signal is a signal for indicating a distance between the convergent point of beam and the recording track in a direction perpendicular to a tangent line of the recording track and an optical axis of the objective lens 205, namely, in the direction of a radius of the magnetic optical disk 101. In this optical pickup device, as indicated by the arrow T in FIG. 1, the objective lens 205 is moved in a direction perpendicular to the optical axis of the objective lens 205 until the tracking error signal becomes 0, namely, tracking servo action is conducted.
Additionally, in an optical pickup device for use in reproduction of an optical disk specially designed for read only or a bit disk such as a compact disk (so-called CD), an integrated type light reception/emission element shown in FIG. 2 has been conventionally employed.
The optical pickup device 210 includes an objective lens 211, optical path bending mirrors 212, 213 and a light reception/emission element 214. Beam emitted from the light reception/emission element 214 is converged to the signal recording surface of the optical disk (CD) 103 through the optical path bending mirrors 212, 213 and the objective lens 211.
The light reception/emission element 214 is composed as an integrated optical block containing a light emission element and a light reception element, as shown in FIG. 3. In this light reception/emission element 214, a second semiconductor substrate 216 is placed upon a first semiconductor substrate 215, and a semiconductor laser chip 217 which is a light emission element is mounted on the second semiconductor substrate 216.
A trapezoidal prism 218 having an inclined face (optical path branching surface) on a side of the semiconductor laser chip 217 is disposed on the first semiconductor substrate 215 forward of the semiconductor laser chip 217. On this optical path branching surface is formed a non-polarization semi-transmission film 218a as a beam splitter. Further, full reflecting film 218b is formed on the top face of the prism 218 and non-polarization semi-transmission film 218c is formed on the bottom face thereof.
Then, the prism 218 reflects beam emitted from the semiconductor laser chip 217 by its optical path branching surface so as to project the beam outside of the light reception/emission element 214. As shown in FIG. 2, the beam emitted from the light reception/emission element 214 is sent to the objective lens 211 through the optical path bending mirrors 213, 212, so that it is converged to the signal recording surface of the optical disk 103 through the objective lens 211.
The reflective beam reflected by the signal recording surface of the optical disk 101 passes through the objective lens 211 and the optical path bending mirrors 212, 213, and then impinges into the prism 218 through the inclined surface of the prism 218 of the light reception/emission element 214. The impinging beam is reflected by the bottom face and the top face of the prism 218. The beam emanates from the prism 218 downward at two positions thereof.
On a top face of the first semiconductor substrate 215 are formed a first and second optical detectors 219a, 219b for receiving beam emanating from the two points of the bottom face of the prism 218.
As shown in FIG. 4, the optical detectors 219a, 219b are divided to four portions, namely, division light reception portions (a, b, c, d), (e, f, g, h) by three division lines extending in parallel in longitudinal direction in the vicinity of the central portion thereof. As a result, a signal RF read from the optical disk 101 is detected by the optical detectors 219a, 219b. If optical detection output signals from the respective division light reception portions are assumed to be Sa, Sb, Sc, Sd, Se, Sf, Sg, Sh, EQU RF=Sa+Sb+Sc+Sd+Se+Sf+Sg+Sh
In the optical detectors 219a, 219b, a differential in detection signal between two division light reception portions of a four-divided sensor element is obtained by so-called push-pull method so as to detect tracking error signal TRK. EQU TRK=(Sa+Se)-(Sd+Sh)
Further, in the optical detectors 219a, 219b, focus error signal FCS is detected based on detection signals from a sensor element in the center and two sensor elements on both sides thereof, in accordance with so-called differential three-division method. EQU FCS={(Sa+Sd)-(Sb+Sc)}-{(Se+Sh)-(Sf+Sg)}
For tracking error signal TRK, so-called TPP (top hold push-pull) method has been proposed to cancel DC offset caused by a move (move of field of view) of the objective lens 211 in a direction perpendicular to the optical axis thereof, accompanied by tracking servo action.
In accordance with the push-pull method, as shown in FIG. 5, the tracking error signal TRK is obtained by comparing intensities of edge portions .beta..sub.1, .beta..sub.2 on both sides of an optical spot .alpha. formed by the beam reflected on the optical disk 103, on the light reception surface of the optical detector 219. When beam emitted from the objective lens 211 is irradiated over the recording track of the optical disk 103, the edge portions .beta..sub.1, .beta..sub.2 on both sides have the same intensity. Then, if the irradiation position for beam emitted from the objective lens 211 is deviated from the recording track, the intensities of the edge portions .beta..sub.1, .beta..sub.2 on both sides become different from each other as shown in FIG. 6. However, if the objective lens 211 is moved so that the field of view is also moved, the beam spot .alpha. on the light receiving surface of the optical detector 219 is also moved, so that DC offset occurs in the tracking error signal TRK.
If the detection output E from the division light reception portion E which receives one side edge .beta..sub.1, of the beam spot .alpha. is considered, a peak of the RF envelop waveform of the detection signal E is changed in a range indicated by the arrow a in FIG. 7, if the field of view is moved. The signal A obtained by passing the RF envelop of the detection output E through a low-pass filter (LPF) is utilized for detection of the tracking error in accordance with the push-pull method. The signal A undergoes a change in offset, in a range indicated by the arrow b in FIG. 7 if the field of view is moved. Thus, if a change in the offset is subtracted from the signal A, the DC offset can be canceled. Here, if such a constant K (&lt;1) in which b=Ka is determined, a signal which cancels the offset is indicated by the signal A-Ka. The same thing can be said for the detection output F from a division light reception portion F which receives a region .beta..sub.2 on the other side of the beam spot .alpha.. As described above, the TPP method obtains the tracking error signal TRK by means of the signal which cancels the offset.
That is, in accordance with the TPP method as shown in FIG. 8, in order to obtain a top hold of the detection output E from the division light reception portion E which receives an edge portion .beta..sub.1 on one side of the beam spot .alpha., a coefficient K is multiplied and then TPP(E) is obtained by subtracting the detection output E from this signal. On the other hand, in order to obtain a top hold of the detection output F from the division light reception portion F which receives an edge portion .beta..sub.2 on the other side of the beam spot .alpha., the coefficient K is multiplied and then TPP(F) is obtained by subtracting the detection output F from this signal. Then, the TPP signal can be obtained by subtracting TPP(F) from the signal TPP(E) (TPP=TPP(E)-TPP(F)).
Further, as a method for eliminating an offset in the tracking error signal in a groove disk having wobble grooves, which is one of the above magnetic optical disk 101, a method for using changes in the wobble component has been proposed.
Meanwhile, in the above optical pickup device for the magnetic optical recording medium, a number of optical devices such as the semiconductor laser 201, the optical detector 209, the beam splitter 203 and the like are mounted individually in an optical block. Steps for production, assembly and adjustment for the optical devices are complicated. Further, reduction of their sizes, improvement of their performance and enhancement of their durability are hard works to overcome.
For an optical pickup device using the above light reception/emission element, its assembly step and adjustment step are easy and reduction of the size, improvement of the performance and enhancement of the durability can be achieved. However, this optical pickup device cannot be used as an optical pickup device in which so-called non-polarization type optical system is utilized and information signal is written or read into/from a magnetic optical recording medium.
Therefore, in order to apply the aforementioned light reception/emission element to the optical pickup device for recording and reproduction for the magnetic optical disk, it is necessary to dispose a parallel flat, half-wave plate 18d between the prism 18 and the first semiconductor substrate 15 as shown in FIG. 9, and further employ P beam splitter (polarized beam splitter) 18e having analyzer function, instead of the non-polarization semi-transmission film 218c which is a beam splitter.
However, if the P beam splitter 18e is utilized only as the beam splitter as described above, in the conventional light reception/emission element, a central value of the incident angles of beam which impinges upon a position of the P beam splitter is as small as about 21.degree.. Thus, a beam splitter formed of multi-layer film cannot be used. Further, the number of parts of the prism 18 increases, so that production step becomes complicated and at the same time, production cost and assembly cost increase.